Final answer:
The correct answer is A. protofilaments; tubulin dimers. Microtubules consist of 13 protofilaments composed of linear chains of tubulin dimers and play a vital role in the cell's structure and dynamics.
Step-by-step explanation:
Microtubules are made up of 13 protofilaments that are linear chains of tubulin dimers. Thus, the correct answer to the student's question is A. protofilaments; tubulin dimers. Microtubules are crucial components of the cytoskeleton, providing structure and facilitating intracellular transport, and are essential during cell division to separate chromosomes.